Apparatus for controlling stock level in a pressurized headbox



Aug. 25, 1959 M. F. GADE 2,901,040

APPARATUS FOR CONTROLLING STOCK LEVEL IN A PRESSURIZED HEAD BOX FiledMay 20, 1957 INVENTOR.

United l tes Patent Marvin FrancisGade, Fullerton, Calif., assignor toKimberly-Clark Corporation, Neeuah, Wis., a corporation of DelawareApplication Ma 20, 1957, Serial N0. 660,175 9 Claims. (Cl. 162-459 Thisinvention relates to paper making machinery and particularly tothelconstruction and operation of a paper making machine headbox andinlet mechanism wherein the stock is to be maintained at a predeterminedlevel. In this connection the stock as used herein will be understood torefer to the dilute aqueous suspension of paper making fiber with orwithout additional paper making materials which is delivered as aflowing stream of fluid to the web forming region of a paper makingmachine by way of the headbox or other stock delivery mechanism.

In the manufacture of a given weight paper on aparticular machine it isfound that there is an optimum depth for the flowing stream of stockwhich is maintained within the headboX. Itis necessary to supply thestock at the web forming region of the machine at a pressure which iscapable of producing an appropriate spouting velocity at the slice. Inorder to obtain such a pressure by conventional means. it is necessaryto provide a relatively high liquid level in the headbox.

However, if the liquid level is relatively high, the velocity of theliquid in the headbox is so slow that the fibers begin settlingl Thisgives poor paper formation. These two r'equirements are inherentlyconflicting, and it necessary to devise a compromise means for obtainingptim m r u v a v In orderto avoid some of these difliculties, it hasbeen proposed to ne est is called a pressurized headborg, thisconstruction, part of the liquid head is r p' ced by a pressurized airpad was to maintain the required total head Thus the level of the stockmay be reduced, thereby increasing the velocity of the liquid in theheadbox sofasi to prevent settling. However, it is essential that thelevel of thestock be accurately controlled so that the velocity of thestock through the headbor; will remain constant or the paper will be ofnonuniform quality. Since the air pad pressure and the stock level aredireetly interrelated, as will be further explained below, it ispossible to control the latter as a function of the former.

ThereforeQit is an objeet of this invention to provide tor the accurateeontroloi stock level in a pressurized headbox of a paper rnachine.Broadly, the invention e sists ofa pneumatic control system whichrelates the a1r pressnre in the headbox to the stock level. By thisarrangement the stocklevel can be accurately controlled at apredetermined optimum level within very narrow limits.

It is also an objeet of theinvention to provide means torcontrollingthestock level at a predetermined optimum level in a pressurized headboxby relating the air pad pressure to the stock ,level in the headbox.

A still further object is to provide a stock level control means whichrapidly responds to changes in the stock level and makes the necessaryadjustments to bring the level back to a predetermined optimum level.

I A further object is to provide means for controlling the level whichis f ully automatic and requires little or as operator attention.

A still further object is to provide a stock level control means whichis economical to install and is economical in operation. s

Other objects will be apparent upon further develop ment of thespecification with reference to the drawing.

In the drawing: p g

The single figure is a diagrammatic view of the stock level controlmeans embodying the invention. v

Refer'ring now to the drawing, there is shown a headbox generallydenoted .by the numeral 10. The headboir is provided with an inlet 11connected to a source of stock which is forced to the headbox 10 by anysuitable pump. Spaced from the inlet is an outlet through which thestock flows onto the web forming wire. A slice.1 3 is positioned at thisoutlet 14 to control this fiow' of the stock onto the web forming'wire15. Extending through the. cover 16 into the chamber 30 is an air inlet17 connected to a pressure source to supply air pressure to the headbox.Secured to the inlet 17 is the air distributing pipe 18 through whichthe air enters into the chamber 30.

Referring now to the system generally, there is shown a differentialpressure transmitter generally denoted by the numeral 19; Thetransmitter is of the diaphragm type including a chamber partitioned bya diaphragm 20, one side 34 of which is connected to the bottom of theheadboit 10 by the duct 31 so as to sense total head pressure therein.The other side 35 of the transmitter 19 is connected by the duct 32 tothe top of the headboit 10 tosense the air pressure therein. 7

It is obvious that the total head pressure (P at, headbox outlet. 14, isequal to the liquid pressure P plus the air pad pressure (P T= L+ A Fromthis relationship, it can be seen that the liquid pressure B is equal tothe total pressure P minus the air pad pressure P L= T- A Furthermore,the liquid pressure is equal to the liquid height multiplied by thedensity of the stock. In other words, the liquid level is directlyproportional to the liquid pressur e. s s s Referring back to thedifferential pressure transmitter, it can be seen that by connecting oneside to the chamber 30 to senseair pressure and the other side to thebottom of the headboX It) to sense total pressure, the differentialpressure across the diaphragm 20 is equal to the liquid head pressurewhich is directly proportional to the liquid level. The diaphragm wouldbe displaced by the differential pressure an amount directlyproportional to the liquid level. For example, if the liquid level wereto increase due to an increased output from the supply pump and the airpressure were to remain constant, the diaphragm would be displaced tothe left by the increased pressure difierential. Conversely, if theliquid level decreased and the air pressure increased, the diaphragmwould be displaced to the right.

While the diaphragm type differential pressure transmitter describedabove is the preferred type of liquid level sensing means, the manometertype differential pressure transmitter is also suitable for thispurpose. As an illustrative example, the manometer type as applied tothis invention would include a U tube, a suitable fluid therein, andmeans for directing the air pad pressure to one fuid surface and thetotal pressure to the other fluid surface. The difference in heightbetween the two snrfaces would be proportional to the stock depth. Theremaining portion of the manometer type device would consist of a fioator other mechanical means used to proportionately activate the pneumaticsignal generator 25 as a function of fluid level in either side of themanometer.

A preferred embodiment, although it is not necessary to the practice ofthe invention, includes means for purging the differential pressuretransmitter 19 with clean or white water on the side 34 that sensestotal pressure. Since this side of the headbox is connected directly tothe transmitter 19 by the duct 31, some of the fibrous material maymigrate thereto. This material, once reaching the transmitter 19, wouldprobably become trapped and collect therein and cause error to beintroduced into the transmitter 19. By using a purge which has an inletpressure only slightly above the total pressure of the headbox, it hasbeen found that the transmitter 19 will be kept free of the fibrousmaterial and at the same time maintain accurate controlof the stocklevel. It is necessary that the purge supply be maintained at a constantpredetermined level only slightly above the total pressure in order notto introduce error into the differential pressure transmitter 19.

Referring now to the drawings, the purge system is generally denoted bythe numeral 12. One way of maintaining the purge system constant is bythe incorporation of a purge flow controller generally denoted by thenumeral 22 into the line. The purge flow controller 22 includes arotameter 23 and a self controlling valve 24. The rotameter is of thetype that may be adjusted to the desired flow rate by means of a needlevalve at its top. The needle valve is mechanically linked to an orificein the rotameter 23 in such a fashion that the area of the orifice maybe varied. The self controlling valve 24 is then adjusted to maintain aconstant pressure drop across the rotameter, corresponding to thedesired flow rate. The self controlling valve 24 continually senses theoutput pressure of the rotameter by means of a pressure tap 36 and alsothe input pressure of the rotameter by sensing the pressure of theliquid flowing through it. By means of its internal linkages including avariable orifice and a diaphragm, these pressure signals are used by theself controlling valve to maintain a constant pressure drop across therotameter. In normal operation the pressure drop is usually maintainedat a constant of 2 p.s.i., thereby controlling the flow rate to apredetermined constant, fixed by the position of the rotameter orifice.

Reference is now made to the output signal from the differentialpressure transmitter 19. The signals are transmitted by appropriatemechanical linkages interconnecting the diaphragm 20 to a pneumaticsignal generator 25 which may be separate or integratted with thedifferential pressure transmitter 19. The pneumatic signal generator 25in turn generates a signal directly proportional to the diaphragmposition which in turn is directly proportional to the stock level. Thesignal may vary between 3 and p.s.i. and the signal at any given instantwill always be in direct proportion to the stock level at that instant.The output signal from the pneumatic signal generator 25 is communicatedto a level recorder controller 26 of any suitable type. One type ofcontroller that is particularly adapted for this use is the reversingtype which emits an output signal inversely proportional to the inputsignal.

It has been found that the pressure signals over the range of stocklevel which it is desired to exercise control must be magnified fromvalues of less than /2 p.s.i. to proportionate pneumatic signals in the3 to 15 p.s.i. range. This magnification is necessary to render thecontroller 26 sensitive to changes in stock level of less than /s" ofwater. The controller 26 is designed toreceive a signal varying between3 and 15 p.s.i. which range corresponds with that of the pneumaticsignal generator. The controller 26 may be of the recording type toprovide a continuous written record of the level of the stock as sensedby the differential pressure transmitter 19. The control section of thecontroller 26 is supplied by p.s.i. air emitted from a suitable pressuresource 33.

This air is. transformed by the control section of the controller 26into the control signal which is conveyed to the pneumatic control valve27. As was explained previously, the control signal is inverselyproportional to the output signal received from the differentialpressure transmitter 19. Thus, if a signal from the pneumatic signalgenerator 25 of a magnitude of 15 p.s.i. were transmitted to thecontroller 26, then a control signal of 3 p.s.i. would be communicatedto the valve 27, assuming the range of the controller to be 3 p.s.i. to15 p.s.i.

The type of controller described above is known as a proportionalcontroller. This method of control is adequate in certain operationssuch as newsprint manufacture where the stock input remains essentiallyconstant during the course of operation. However, in certain types ofoperations where the grade of the paper is periodically changed andcorresponding changes in stock input are necessary, it has been foundthat a proportional control is not adequate. Due to an inherentcharacteristic of proportional control, offset, or difference betweendesired control point and actual control point, is introduced whensignificant changes in stock input are made.

In view of this it has been found that if the invention is used undervarying stock input conditions, the incorporation of a reset featureinto the controller is highly desirable. The reset feature is amechanical means for eliminating offset which tends to occur undervarying stock input conditions.

Referring now to the pneumatic control valve 27, it may be of anysuitable type. The valve is installed on a bleed line 28 from an air padsupply line so as to bleed controlled amounts of air from the aid padsupply pressure. As was explained previously, the stock level is sointerrelated with the air pad pressure that it may be controlled byvarying the latter. The air pad supply line is equipped with a constantpressure air supply (not shown) and may also be provided with a manualbleed valve 29. By means of the manual bleed valve 29, the air padpressure may be adjusted to bring the system within the scale on thecontroller 26.

One type of control valve that is particularly adapted for this use isthe air to open. By using the air to open control valve 27 fouling ofthe air showers is eliminated in the event of failure of the controlair. If the control air 33 fails, the control valve is closed and theair pressure will increase in the headboX, thus lowering the stock levelso as to give the operator prompt warning of the failure. If an air toclose valve were used and the control air were to fail, the headboxwould fill with stock which would foul the air showers and otherinternal piping and fittings before being noticed. While both types ofcontrol valves may be used, the air to close type requires more operatorattention, so the preferred embodiment of the invention includes the airto open valve rather than an air to close valve.

The operation of the system will now be described. If the stocklevel'were to drop from the predetermined optimum level, the resultantdecrease in hydraulic pressure would be transmitted to the differentialpressure transmitter 19. Due to the drop in the stock level thediaphragm would be displaced toward the liquid side of the transmitter19, causing a decreased signal to be emitted by the pneumatic signalgenerator 25 to the level recorder controller 26. The level recordercontroller being a reversing controller communicates an increasedcontrol signal to the pneumatic control valve 27. The pneumatic controlvalve in turn will partially open, thus bleeding air from the air padsupply line. The bleeding will result in lower air pressure in theheadbox and allow the stock level to rise to the predetermined optimumlevel. Conversely, should the level of the stock rise above the optimumlevel, then the diaphragm would be displaced toward the air side of thetransmitter 19, causing an increased signal to be emitted by thepneumatic signal generator 25. The varione components of the inventionmay be placed in any desired position with the exception of thedifferential pressure transmitter. It has been found desirable to placethe transmitter at some point below the headbox and thereby allow thebleeding of any air in the transmitter 19 to the headbox. The increasedpressure due to the placing of the transmitter 19 below the headbox maybe eliminated by the proper adjustment of the diflerential pressuretransrnitter.

It is obvious that certain modifications may be made within the scope ofthe claims without departing from the spirit of the invention.

I claim:

1. A system for controlling the stock level in an air pressurizedheadbox of a paper making machine comprising means for sensing thedifference between the stock pressure at the outlet of said headbox andthe air pressure within said headbox, and a valve for varying the airpressure in said headbox, said valve being actuated by said means.

2. A system for controlling the stock level in an air pressurizedheadbox of a paper making machine comprising a diiferential pressuretransmitter having a flexible diaphragm, one side of said diaphragmacted upon by the stock pressure at the headbox outlet, the other sideof said diaphragm being acted upon by air pressure within said headbox,and a valve for varying the air pressure in said headbox, said valvebeing controlled by said differential pressure transmitter.

3. A system for controlling the stock level in an air pressurizedheadbox of a paper making machine, comprising a differential pressuretransmitter having a flexible diaphragm, one side of said diaphragmbeing acted upon by the stock pressure at the headbox outlet, the otherside of said diaphragm being acted upon by air pressure in said headbox,a pneumatic signal generator actuated by said differential pressuretransmitter, and a valve for varying the air pressure in said headbox,said valve being controlled by the pneumatic signal generator.

4. A system for controlling the stock level in an air pressurizedheadbox of a paper making machine, comprising a differential pressuretransmitter having a flexible diaphragm, one side of said diaphragmbeing acted upon by the stock pressure at the headbox outlet, the otherside of said diaphragm being acted upon by the-air pressure within saidheadbox, a valve for varying the air pressure within said headbox, andmeans for purging said differential pressure transmitter of foreignmaterial.

5. A system for controlling the stock level in an air pressurizedheadbox of a paper making machine, comprising a differential pressuretransmitter having a flexible diaphragm, one side of said diaphragmbeing acted upon by air pressure within said headbox, the other side ofsaid diaphragm being acted upon by the total pressure in said headbox,means for purging said second side of foreign material, a pneumaticsignal generator controlled by the position of said diaphragm, and avalve for varying the air pressure in said headbox, said valve beingactuated by the signals from said pneumatic-signal generator.

6. A system for controlling the stock level in an air pressurizedheadbox of a paper making machine, comprising a differential pressuretransmitter having a flexible diaphragm, one side of saiddiaphragmfbeing acted upon by the air pressure within said headbox, theother side of said diaphragm being acted upon by the total pressurewithin said headbox, means for purging said second side of foreignmaterial, a pneumatic signal generator controlled by the position ofsaid diaphragm, a valve for varying the air pressure within saidheadbox, and controller means for communicating signals from saidpneumatic signal generator to said valve, said valve being controlled bysaid signals.

7. An air pressurized headbox for delivering stock to the web formingregion in a paper making machine, comprising a closed chamber havingspaced stock inlet and stock outlet ports positioned below the level ofthe stock within said chamber, means for maintaining air pressure insaid chamber to act upon said stock, a differential pressure transmitterhaving a flexible diaphragm therein, one side of said diaphragm beingacted upon by air pressure within said chamber, the other side of saiddiaphragm being acted upon by the stock pressure at said outlet port,and a valve for varying the air pressure Within said chamber, said valvebeing controlled by the position of said flexible diaphragm.

8. An air pressurized headbox for delivering stock to the web formingregion in a paper making machine, comprising a closed chamber havingspaced stock inlet and stock outlet ports positioned below the level ofthe stock within said chamber, means for maintaining air pressure insaid chamber to act upon said stock, a differential pressure transmitterhaving a flexible diaphragm therein, one side of said diaphragm beingacted upon by the air pressure within said chamber, the other side ofsaid diaphragm being acted upon by the stock pressure in said chamber,and a valve for varying the air pressure in said chamber, said valvebeing controlled by the differential pressure of said diaphragm.

9. A pressurized headbox used for delivering stock to the web formingregion in a paper making machine comprising a closed chamber, havingspaced stock inlet and stock outlet ports positioned below the level ofthe stock in the chamber, means for maintaining air pressure in saidchamber to act upon said stock, a differential pressure transmitterhaving a flexible diaphragm therein, one side of said diaphragm beingacted upon by the air pressure in said chamber, the other side beingacted upon by the combined liquid and air pressures in said chamber, anda valve for varying the air pressure in said chamber, said valve beingcontrolled by the position of said flexible diaphragm.

References Cited in the file of this patent UNITED STATES PATENTS2,309,228 Waterman J an. 26, 1943 2,381,286 Hornbostel et al. Aug. 7,1945 2,386,108 Gess et a1 Oct. 2, 1945 2,736,246 Crittenden et al. Feb.28, 1956

